The attributes allowing an embedded video (e.g. Vimoe video) to be displayed in full screen (e.g. webkitallowfullscreen, mozallowfullscreen, allowfullscreen) are removed when the embedded code is used in a Content block widget.
Here is a sample code which can be placed in the HTML mode of the Content block widget:
After the changes are saved or the user switches to Design mode, the allowfullscreen attributes are removed. This is the reason why when accessing the page with the embedded video on the frontend it cannot be displayed in full screen mode.